The most basic tube has two elements: a cathode (also known as the emitter), and an anode (also known as the plate). The cathode is connected either to ground or a negative voltage; the plate is connected to a high positive voltage. As the cathode is heated, electrons jump from the cathode to the plate.
In a basic triode tube, a control grid sits between the cathode and plate. A voltage applied here either encourages or inhibits the flow of electrons from cathode to plate. In this way, a small voltage (such as an audio signal) applied to the control grid results in a much larger change in voltage at the plate, causing that input signal to be amplified.
A pentode tube has two additional elements: a screen grid and a suppressor grid. In a typical design, the screen grid amplifies the signal further, while the suppressor grid blocks the “secondary emission” of electrons bouncing off the plate and returns them to the cathode, attenuating the output.
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